Forces between Mica Surfaces, Prepared in Different Ways, Across

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Forces between Mica Surfaces, Prepared in Different Ways, Across Aqueous and Nonaqueous Liquids Confined to Molecularly Thin Films Susan Perkin,† Liraz Chai,‡ Nir Kampf,‡ Uri Raviv,‡,| Wuge Briscoe,† Iain Dunlop,†,⊥ Simon Titmuss,† Minseok Seo,§ Eugenia Kumacheva,*,§ and Jacob Klein*,†,‡ Physical and Theoretical Chemistry Laboratory, UniVersity of Oxford, Oxford OX1 3QZ, U.K., Materials and Interfaces Department, Weizmann Institute of Science, RehoVot 76100, Israel, and Department of Chemistry, UniVersity of Toronto, Toronto, Ontario M5S 3H6, Canada ReceiVed NoVember 16, 2005. In Final Form: April 17, 2006 We have measured normal and lateral interactions across a range of different liquids between mica surfaces using a surface force balance (SFB). The mica surfaces were prepared either by melt cutting using Pt wire and standard procedures in our laboratories or by tearing sheets (that had not been exposed to Pt) off from a freshly cleaved sheet of mica. AFM micrographs revealed the substantial absence of Pt nanoparticles on the melt cut and torn-off mica surfaces. Normal-force versus surface-separation (D) profiles and shear force versus D measurements for purified water (no added salt), for concentrated aqueous NaCl solutions, and for cyclohexane revealed that in all cases the behavior of the highly confined liquids between melt-cut and between torn-off mica sheets was identical within experimental scatter. These results demonstrate directly that interactions measured between melt-cut mica surfaces as routinely prepared using established procedures in our laboratories and in other laboratories are free of the effect of any Pt contamination.

Introduction Since its invention in the late 1960s, the surface force apparatus or surface force balance (SFB) has been extended1-3 to determine interactions between mica or coated mica surfaces across vacuum, air and in liquid media with unique sensitivity and resolution. These studies have included static and dynamic properties of the confined species (whether pure liquids, solutions, or surfaceattached molecules) as well as frictional forces between the confining surfaces.4-7 The essential features are the use of cleaved, molecularly smooth solid surfaces (the crystallographic mica [100] surfaces, which may be coated if desired) as the interacting substrates and the optical interferometric method that enables direct measurements of absolute separations (with an accuracy of some ((0.1-0.3) nm) between them. In these, the SFB technique differs qualitatively from scanning probe methods (such as AFM) because these can measure only relative displacements between surfaces whose topology, moreover, is often not well defined at the molecular level. The accuracy and interpretation of results using the SFB, in particular those pertaining to interactions and friction across simple liquids where the surface separations of interest may be on the order of a few nanometers or less, rely on the mica surfaces * Corresponding authors. E-mail: [email protected]. E-mail: [email protected]. † University of Oxford. ‡ Weizmann Institute of Science. § University of Toronto. | Present address: Institute of Chemistry, Hebrew University of Jerusalem, Israel. ⊥ Present address: Max Planck Institut fu ¨ r Metallforschung, Heisenbergstrasse 3, 70569 Stuttgart, Germany. (1) Tabor, D.; Winterton, R. H. S. Proc. R. Soc. London, Ser. A 1969, 312, 435-450. (2) Israelachvili, J.; Adams, G. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975-1001. (3) Klein, J. Nature 1980, 288, 248-250. (4) Israelachvili, J.; McGuiggan, P. M. Science 1988, 240, 189-191. (5) Klein, J.; Perahia, D.; Warburg, S. Nature 1991, 352, 143-145. (6) Granick, S. Science 1991, 253, 1374. (7) Schorr, P. A.; Kwan, T.; Kilbey, M.; Shaqfeh, S. G.; Tirrell, M. Macromolecules 2003, 36, 389-396.

being reproducibly molecularly smooth and uniform. It has long been known that a film of thickness of a few angstroms rapidly forms on mica because of the adsorption of a hydrocarbon/water complex from the ambient atmosphere on exposure of the freshly cleaved surfaces.8,9 Such layers are water-soluble and when immersed in water are removed to expose the essentially virgin smooth mica surface.1,2,10,11 Over the past few years, however, it has been reported that use of the traditional melt-cutting technique for preparing mica for surface force experiments could, in certain circumstances, result in nanoparticulate contamination on mica.12-16 AFM images of mica surfaces that were melt cut or exposed to a hot platinum wire revealed particles of height h ≈ 2-10 nm and lateral dimensions of 20-100 nm, which were initially assumed to be molten mica12 but were subsequently shown to be platinum.13 Clearly, the presence of such nanoparticles on the surface could significantly change the nature of the forces between them at separations of D < h. This applies especially when liquids are confined between surfaces down to nanometer and even subnanometer thin films, though it is unlikely to affect measurements at separations of D . h (as, for example, with surface-attached polymer layers7,17). Since the possible presence of nanoparticles on mica was first reported in 1999,12 a number of studies have addressed this issue directly. Kohonen et al. pointed out some of the conditions for minimizing or eliminating such Pt particles,13 noting in particular the importance of allowing adequate air flow past the surfaces (8) Poppa, H.; Elliot, G. Surf. Sci. 1971, 24, 149-163. (9) Dowsett, M. G.; King, R. M.; Parker, E. H. C. J. Vacuum Sci. Technol. 1977, 14, 711-717. (10) Pashley, R. J. Colloid Interface Sci. 1981, 83, 531-546. (11) Raviv, U.; Klein, J. Science 2002, 297, 1540-1543. (12) Ohnishi, S.; Hato, M.; Tamada, K.; Christenson, H. Langmuir 1999, 15, 3312-3316. (13) Kohonen, M.; Meldrum, F.; Christenson, H. Langmuir 2003, 19, 975976. (14) Heuberger, M.; Zach, M. Langmuir 2003, 19, 1943-1947. (15) Lin, Z.; Granick, S. Langmuir 2003, 19, 7061-7070. (16) Israelachvili, J.; Alcantar, N.; Maeda, N.; Mates, T.; Ruths, M. Langmuir 2004, 20, 3616-3622. (17) Taunton, H.; Toprakcioglu, C.; Fetters, L.; Klein, J. Macromolecules 1990, 23, 571579.

10.1021/la053097h CCC: $33.50 © 2006 American Chemical Society Published on Web 06/07/2006

Forces between Mica Surfaces

during the melt-cutting in the laminar flow hood. In a detailed study,15 Lin and Granick showed that such nanoparticles were persistently present on mica sheets as melt-cut in their laboratory. In contrast, Israelachvili et al. showed16 in a systematic study that the melt-cutting procedure as employed by them resulted in surfaces where essentially no nanoparticles at all could be detected (“less than 1 per 100 µm2”, which in any case was probably not Pt) except near the edges of the melt-cut samples, far from the region of interest where forces between mica surfaces are actually measured. They tentatively attributed the difference between their results and those of Lin and Granick to the rather long (6 cm, 312-500 µm diameter) Pt wire used to cut the mica sheets in the latter study compared with an 8 mm length of Pt wire (e250 µm thick) in the Israelachvili laboratory, resulting in the extensive Pt nanoparticle deposition in the Lin/Granick study. In another study,18 Raviv et al. directly compared the normal and lateral forces between mica surfaces across water both for the case of melt-cut mica (as implemented in their laboratory using ∼1-1.5-cm-long, 125-µm- or 250-µm-diameter Pt wires) and for the case of so-called torn-off mica. In the latter case, the thin, molecularly smooth mica sheets were obtained by detaching them from a larger cleaved mica sheet using tweezers, that is, without any use of a hot Pt wire as used in melt cutting. The results in both cases were identical, and AFM micrographs revealed that the melt-cut mica and the non-melt-cut mica were both free of nanoparticle contamination at the contact areas studied and away from any melt-cut edge. The situation, therefore, is that in one or two laboratories measuring surface forces14,15 the mica melt-cutting procedures clearly led to the presence of nanoparticles in the regions of surfaces being measured; in others the procedures followed were such that nanoparticle contamination, as revealed by AFM16,18 and controls18 using mica prepared without Pt melt-cutting, was effectively absent. It is important to emphasize that the areas scanned by AFM are limited and that there is a possibility of sweeping particles aside during scanning.12 For these reasons, the absence of nanoparticles revealed by AFM is not conclusive evidence that they are entirely absent and will not affect the results. Likewise, the presence of individual Pt atoms on the mica surfaces at the submonolayer level may not be detected by AFM or by the XPS methods used13,15,16 and could also influence measured surface forces. The most direct way to examine whether such artifacts are absent, or have no measurable effect, is to compare, for the same liquids and under the same conditions, the measured interactions between melt-cut mica surfaces with those measured between non-melt-cut (e.g., torn-off) mica surfaces, which by construction cannot be contaminated by Pt in any form. The present study was initiated with precisely this goal. Here we extend our earlier report18 to examine systematically whether surface force measurements using melt-cut mica surfaces as prepared in our laboratories are identical to those using nonmelt-cut mica for generically different liquid systems ranging from pure water to high-salt-concentration aqueous electrolytes to a typical simple organic liquid. Results are given from experiments carried out by different researchers independently in three different SFB laboratories (Oxford University, Weizmann Institute, and Toronto University). (Data are identified in the Figure captions as either Oxford and/or Weizmann and/or Toronto according to the laboratory or laboratories where it was taken.) In the following section, we describe procedures used in our laboratories for preparing both melt-cut and torn-off mica sheets and the characterization of their surfaces. We then compare in (18) Raviv, U.; Perkin, S.; Laurat, P.; Klein, J. Langmuir 2004, 20, 53225332.

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Figure 1. AFM images of a mica sheet with Pt nanoparticles intentionally deposited on the surface. The Pt wire was placed upstream from the mica in the laminar flow, as shown in the diagram inset of A, and held there while passing a current of 2.7A for 30 s. Images were recorded using a Nanoscope III in tapping mode at positions indicated for each micrograph. Particles on image A, a region 1.5 ( 0.5 mm from the wire, were typically 2-8 nm high and 20-100 nm in diameter with a particle density of ∼15-20 µm-2. For B (taken at a higher magnification), a region ca. 4 mm from the wire, they were around 2-6 nm high and 5-20 nm in diameter with a particle density of ∼10 µm-2 (Oxford).

detail results for several different SFB experiments using both types of mica across purified water (no added salt), across concentrated aqueous NaCl solution, and across a simple model organic solvent (cyclohexane). Both normal and shear forces are considered because the latter may be expected to be more sensitive to nanoparticulate contamination. We also relate briefly to a different approach for creating non-melt-cut mica surfaces, the so-called tape-peeling approach.19,20 The implications of our results for the interpretation of earlier surface force experiments are considered in the Discussion and Conclusions section. Methods and Materials Traditionally, mica sheets have been prepared for SFB experiments by melt-cutting 1-5-µm-thick facets using hot Pt wire. These samples are then placed on a freshly cleaved mica backing sheet, backsilvered, and then lifted with tweezers (a process facilitated by the melted sample edges) and glued, silver side down, onto cylindrical glass lenses for the experiment. The precise details of this procedure are crucial to the discussion regarding the presence or absence of Pt nanoparticles. The mica thickness, ease of airflow past the surfaces in the laminar flow hood, length and diameter of wire, heating current, cutting rate and size of piece cut will each have a significant effect on the amount of Pt deposited, as has been considered in detail in earlier studies.13,15,16 Below are descriptions of two mica preparation procedures used in our laboratories; one is based on the traditional melt-cutting method, and the other eliminates the use of Pt wire entirely. Both methods produce pairs of mica sheets from a single facetssheets of mica that are molecularly smooth on both sides and have uniform thicknesssleading to a symmetrical interferometer, in contrast to the method of “tape peeling”19,20 (discussed later). Preparation of Melt-Cut Mica. Mica is cleaved to create 1-3µm-thick facets (exposing a single crystallographic [100] plane on each side) and is then supported using brass blocks, taking care not to allow any object to obscure the passage of air flow from the back of the hood past the exposed mica to be cut. The mica is oriented such that when the first cut is made the hot Pt wire will be downstream of the mica in the airflow (i.e., the cut will be perpendicular to the air flow, and the flow will carry any Pt evaporating from the wire away from the mica to be used). This configuration is shown in the inset of Figure 2. The Pt wire used for these experiments is ∼1-1.5 (19) Frantz, P.; Salmeron, M. Tribol. Lett. 1998, 5, 151-153. (20) Zhu, Y.; Granick, S. Langmuir 2003, 19, 8148-8151.

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Figure 2. AFM images (Oxford), at two magnifications as shown, of a mica sheet melt cut according to the procedures in our laboratories. No particles are detected in the region of the micrographs, and none were observed when imaging other parts of the mica surface (away from the melt-cut edge itself). The cutting method (downstream melt cutting) is indicated schematically in the inset of A and is described in the text. Similar images were recorded using both tapping and contact mode AFM, and the measured rms roughness is ∼0.2 nm.

Figure 3. AFM image (Oxford) of torn-off mica (Nanoscope III in tapping mode), prepared as shown in the inset of A and as described in the text, showing a particle-free surface. The scan area is 10 × 10 µm2, and no particles were observed via AFM anywhere on this or on other similarly prepared sheets. (B) Schematic of the freshly cleaved base sheet on which three “steps” (created using 40-80µm-thick mica rectangles freshly cleaved on both sides) are adhered, onto which the torn-off facets are laid with a small overlap to facilitate subsequent peeling off.

cm long with a diameter of either 125 µm (1.5 cm long) or 250 µm (∼1 cm long) and is heated using currents in the range of 2.0-2.7 A (125 µm) and 7-8 A (250 µm). After the first straight cut, the wire is removed, and the mica is rotated by 90° so that the second (and usually final) cut can also be made downstream. The mica piece, now held with tweezers, is then turned over and placed onto the freshly cleaved backing sheet. Backing sheets were generally exposed to air in the laminar flow hood for less than 1 h, or often much less, before the mica pieces were laid down. The cutting time is as short as possible: 2-10 s/cut. Using this method, single large pieces of mica (3-10 cm2) are prepared and are cut into smaller pieces using a scalpel immediately prior to use. In some of the melt-cut mica experiments described, the downstream procedure was not followed or was only partially followed (as described in the Results section and in the Figure captions). In all cases, it was found to be important for contaminant-free experiments that the mica sheets be free of visible air pockets and be strongly adhered to the backing sheet. This was almost invariably the case for downstream-cut mica. On those occasions when contamination was found to be present, it could often be attributed to the absence of these conditions. Preparation of Torn-Off Mica. Facets of 1-3-µm-thick mica are formed by cleaving from larger, thicker sheets and are supported by blocks as indicated in the inset to Figure 3a, and the whole facet (indicated by a single, step-free bright color) is carefully torn away using tweezers and laid on the backing sheet. Because torn mica has no previously melted edges, it is very difficult to pick up when the whole piece is adhered to the backing sheet. To avoid this problem, one or more steps are created on the backing sheet to support one of the edges of the sample piece, as indicated in Figure 3b. A step consists of another piece of mica, freshly cleaved on both sides and 40-80 µm thick and adhered to the freshly cleaved backing sheet. This creates a “lip” across which the torn-off piece is lain with a 1-2 mm overlap supported by the step, which allows the piece to be subsequently lifted with tweezers. Alternatively, the torn-off facet may be laid with an edge dangling off the freshly cleaved base sheet to facilitate subsequent lifting. Large pieces prepared in this manner can be divided into smaller pieces using a scalpel immediately prior to use. Effect of Short Exposure on Freshly Cleaved Mica Surfaces. We examined the issue of short time exposure of freshly cleaved mica to ambient air (as relevant for the tape-peeling method20,21) by carrying out a “partial cleavage” experiment similar to that reported in the early surface-force studies1,2 but for much shorter exposure times than the hour or so in the previous reports. A thin sheet of mica

(8-10 µm) was half-silvered on both sides, and then a cleavage was initiated at one edge and the sheet was cleaved halfway. The cleavage was held open for times of 1 min and longer by clamping with brass blocks in a laminar flow hood, following which the partially cleaved sheet was closed up again. This procedure is shown schematically in Figure 9b. Fringes of equal chromatic order (FECO) of white light transmitted through the closed sheet were then measured across the region containing the cleavage bifurcation line to determine whether any contaminant layer had formed on the cleaved regions over the short time of their exposure to air. AFM Imaging. AFM imaging was carried out using a Nanoscope III, generally in tapping mode. Scans were performed on many different areas of each sample examined, with scan sizes between 1 × 1 and 10 × 10 µm2. SFB Measurements. The SFB technique has been described.1,2,22,23 Single-facet mica sheets, which are crystallographically smooth on both sides, were prepared by melt cutting or by tearing as described above and deposited onto base sheets. These were half-silvered and subsequently glued onto cylindrical glass lenses and then mounted in a crossed configuration inside the apparatus. Glues used were EPON 1004 (Shell) resin for the aqueous systems or sym(diphenyl carbazide) (analytical grade, Aldrich, Canada) for the cyclohexane experiment. After calibrating the system in air, pure water or cyclohexane was added, and the normal and shear forces between the mica surfaces were measured as a function of surface separation. In the case of experiments in water or in aqueous salt solution, only when the osmotic repulsion in salt-free water is followed by a van der Waals jump-in to adhesive contact (to a separation smaller than in air contact, see below) is the system considered clean and the experiments continued (or salt added to the required concentration for the aqueous salt solution experiments). The jump-in is due to a spring instability whenever ∂F/∂D g Kn, the normal spring constant (ca. 150 N/m in all of these experiments). Following the normalforce measurements, shear forces were measured at different loads, surface separations, and shear protocols. Materials. Mica is of the ruby muscovite variety, either V-2 special grade or grade 1 (S&J Trading Inc., New York) or grade 2 (Mica Corporation, New York). Conductivity water is prepared by passing filtered tap water through a Millipore water-purification apparatus consisting of a RiOs5 and Milli-Q gradient A10 system. The water has resistivity greater than 18.2 MΩ‚cm and a total organic content of 2-4 ppb. The NaCl was 99.999% pure (Aldrich), treated

(21) Zhu, Y.; Granick, S. Phys. ReV. Lett. 2004, 93, 096101 1-4.

(22) Klein, J. J. Chem. Soc., Faraday Trans. 1 1983, 79, 99-118. (23) Klein, J.; Kumacheva, E. J. Chem. Phys. 1998, 108, 6996-7008.

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with UV light for ∼1 h before use, or 99.886% pure (Fluka certified standard). Cyclohexane was HPLC grade (99.9+%, Aldrich, Canada) and used as received from a freshly opened bottle. (Earlier studies23 showed that results using cyclohexane from such freshly opened bottles were identical to those using cyclohexane that had been double distilled; cyclohexane experiments were completed within 1 day.) All glassware was cleaned in piranha solution and then in water and ethanol. All tools (for cleaving as well as for the SFB experiments) and all parts of the apparatus in contact with the solution were either glass or stainless steel, passivated in 30% HNO3 and then rinsed in water followed by ethanol. Parts were then sonicated in toluene, then in ethanol, and then (for the aqueous experiments) in water. For the cyclohexane experiments, a small beaker of P2O5 was placed in the SFB chamber to maintain dry conditions.

Results Figure 1 shows AFM images of a mica sheet upon which Pt nanoparticles were intentionally deposited, obtained by holding a hot Pt wire upstream from the mica in the laminar flow hood for 30 s, as shown in the inset diagram. Particles are clearly visible on the mica, and their density and characteristics depend as expected on the distance from the wire (typically 2-8 nm in height and 20-100 nm in diameter and for a particle density of ∼15-20 µm-2 for regions ca. 1.5 mm from the wire (Figure 1a) and around 2-6 nm in height and 15-20 nm in diameter for a particle density of ∼10 µm-2 for regions 4 mm from the wire). These values are comparable, though covering a wider range, to those reported in other studies where Pt nanoparticles were deposited during the mica melt-cutting process.12-15 Figure 2 (main) again shows AFM images of 10 × 10 and 1 × 1 µm2 scans, this time, in contrast, of mica that has been (downstream) melt cut, prepared as described above and as shown in the inset of Figure 2. No nanoparticles are visible at either magnification, nor were any observed elsewhere on the mica surface away from the molten edge itself, either on the same sample or on different samples prepared similarly. Other studies (not shown here) where mica was cut with the hot Pt wire partially downstream and partially parallel to the air flow occasionally show nanoparticles (densities in the region of 1 µm-2) on the mica surface, close to the melt-cut edges (though none in the central region where measurements are made, in agreement with ref 16). This confirms that in order to maximize the elimination of Pt nanoparticles everywhere on the surface, not just from the regions where measurements are made, it is desirable to cut as far as possible downstream. (This is in line with the observation by Kohonen et al.13 that the draft in the laminar flow hood can be very efficient at carrying away the evaporated Pt atoms.) Figure 3A shows a 10 × 10 µm2 AFM image of torn-off mica, prepared as described above and as indicated in the inset to the Figure. As one expects, there are no Pt nanoparticles on the surface. This is clearly the most certain way to avoid the presence of Pt altogether while still achieving two mica pieces of identical thickness. Figure 4 shows the normal-force profiles F(D)/R at separations D between two mica surfaces (mean radius of curvature R ≈ 1 cm) across conductivity water. The data shown in Figure 4 is taken in several separate experiments in two different laboratories using mica sheets prepared independently by five of the coauthors. It includes measurements with pairs of mica sheets that had been either torn off or fully downstream melt-cut or partially downstream melt-cut (then scalpel cut to use sheets far from the melt edges). For all profiles taken, the form of the osmotic repulsion between the surfaces, the surface separation (Dj, in the range of 4 ( 2 nm) from which jumps-in occur, and, for some of the experiments, the solid-liquid surface tension (γ, deduced

Figure 4. Normal-force profiles F(D)/R (where R ≈ 1 cm is the mica curvature) between curved mica surfaces in a crossed-cylinder configuration across conductivity water as a function of surface separation D (measured on approach of the surfaces). Mica sheets were prepared and data were taken by five of the coauthors independently in two laboratories. Profiles using torn-off mica are shown as solid symbols (Oxford), and those taken using melt-cut mica (both fully and partially downstream melt-cut) are shown as open symbols and as crosses (Weizmann and Oxford). In every case, the surfaces jump together on approach, from separations of D ) Dj ) 4 ( 2 nm into contact at a surface separation in the range of (0.5-1) ( 0.3 nm closer in than their air-contact position. (The normal spring constant is ca. 150 N/m for all experiments.) The lower and upper dashed curves are calculated from a nonlinear DLVO expression (according to the algorithm of Chan et al.58) using effective concentrations of 1.0 × 10-5 and 2 × 10-6 M monovalent ions and surface potentials of 155 and 130 mV, respectively. The solid curve is from the study on conductivity water of ref 25. The surfaces jump apart upon separation, and from the pull-off force required to separate the surfaces, measured for torn-off mica (solid diamonds and inverted triangles) and melt-cut mica (open circles, diamonds, and upright triangles), the solid-liquid surface tension was calculated to be γ ) -2.5 ( 0.5 mN/m, with no significant variation between melt-cut and torn mica samples.

from the pull-off force, caption to Figure 4) were the same within the scatter (and well within the range of literature values24,25) whether torn mica or melt-cut mica was used. Moreover, in all cases (both torn off and melt-cut) the surfaces jumped in across the conductivity water to a separation of ca. 0.8 ( 0.5 nm closer in than their air-contact position. All D values in our experiments in water or aqueous salt solutions are relative to D ) 0 defined by this contact separation in pure (no salt added) water. Small variations in the magnitude and decay length of the osmotic repulsions observed for different profiles may be attributed, as considered in the Discussion, to variations in the effective salt concentration in the conductivity water (which is due to ions leaching from glassware) or in the surface potential of the mica, both of which can vary slightly from experiment to experiment. The presence of Pt nanoparticles, if any, between mica sheets across liquids at separations closer than the nanoparticle height h might be expected in particular to affect the shear forces between them as they slide past each other. To examine this in salt-free (conductivity) water, we carried out additional shear experiments (independently in two different laboratories) similar to those reported earlier:18,26 The surfaces were positioned about 10 nm from each other, and while the top surface was moved back and forth laterally parallel to the lower surface, the two surfaces were allowed to approach very slowly under thermal drift until they jumped into contact. The shear forces between them were (24) Raviv, U.; Laurat, P.; Klein, J. J. Chem. Phys. 2002, 116, 5167-5172. (25) Pashley, R. J. Colloid Interface Sci. 1981, 80, 153-162. (26) Raviv, U.; Laurat, P.; Klein, J. Nature 2001, 413, 51-54.

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Figure 5. Time traces (lower trace of each pair) of the shear forces Fs transmitted between the mica surfaces a distance D apart (as indicated on the traces) across conductivity water as the top surface moves laterally back and forth (upper trace of each pair) past the bottom one. Surfaces jump together into contact from separations D ) Dj (arrows) in the range of 4 ( 2 nm. (A) Melt-cut mica (Weizmann). (B) Fully downstream melt-cut mica (Weizmann). (C) Torn-off mica (Oxford). On the RHS are shown the frequency (ω)dependent Fs(ω) values corresponding to the shear force traces prior to and during the jump-in, with arrows indicating the response at the respective drive frequencies ω0. (The peaks at ca. 2 Hz for Fs(ω) of traces A and B arise from building oscillations (Weizmann Institute) that are absent from Fs(ω) for trace C (Oxford)). The magnitude of Fs(ω0) is in all cases within (30 nN of its mean value at large D (> ca. 100 nm), as indicated by the dotted Fs(ω) for trace C taken at D ) 150 nm, indicating that the net shear force prior to jump-in is below the resolution of 30 nN.

monitored directly throughout. Figure 5A-C shows these shear force versus time traces for partially downstream-melt-cut mica (A), fully downstream-melt-cut mica (B), and torn-off mica (C). In all three cases, the behavior is indistinguishable: no shear forces higher than the noise level are observed as the surfaces drift slowly toward each other while moving laterally back and forth past each other, up to the point J (D ) Dj) where they jump into contact, and no forces higher than the noise level are observed during the jump-in itself. Forces were measured (again independently in two laboratories) between the differently prepared mica sheets across concentrated aqueous salt solutions. The normal and shear interactions between mica surfaces in NaCl solutions prepared to concentrations of 0.06 ( 0.01 M are shown in Figure 6 for both torn-off and melt-cut mica samples. The salt concentration is above the threshold required to observe a hydration repulsion force;27 consequently, the surfaces do not reach a primary minimum on approach. The normal forces between the torn-off and melt-cut mica surfaces, shown in Figure 6A, are identical within the scatter. The shear force traces presented in Figure 6B and C, revealing (27) Pashley, R. AdV. Colloid Interface Sci. 1982, 16, 57-62.

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Figure 6. (A) Normal-force profiles F(D)/R between curved mica surfaces in a crossed-cylinder configuration across aqueous 0.06 ( 0.02 M NaCl as a function of surface separation D (measured on approach of the surfaces). Each symbol style represents a different force run, with profiles using torn mica indicated by filled symbols (Oxford) and those using melt-cut mica (not fully downstream) indicated by open symbols (Weizmann). The dashed line is a fit using a nonlinear DLVO expression58 with a monovalent ion concentration of 0.06 M, surface potential of ψ ) 50 mV, and an additional repulsive term F/2πR ) Eh exp(-D/Dh) using 2πEh ) 0.6 N m-1 and Dh ) 0.2 nm. The dotted line corresponds to literature profiles using melt-cut mica.59 B and C show (for the same 0.06 M NaCl solution) the response of the bottom mica surface (lower trace of each pair) to an applied back-and-forth lateral motion of the top mica surface (upper trace) across a film of thickness D as shown, using melt-cut mica and torn mica, respectively. On the right are shown the frequency (ω)-dependent Fs(ω) values corresponding to the prior shear force traces, with arrows indicating the response at the respective drive frequencies ω0. (The peaks at ca. 2 Hz for Fs(ω) of B arise from building oscillations (Weizmann Institute) that are absent from Fs(ω) for trace C (Oxford)). The magnitude of Fs(ω0) is in all cases within (30 nN of its mean value at large D, as indicated by the dotted Fs(ω) for trace C taken at D ) 5.1 nm, indicating that the net shear force prior to jump-in is below the resolution of 30 nN.

the very weak shear forces even when the surfaces are compressed to a nanometer or less, confirm the extremely efficient lubricating effect of the trapped hydrated ions reported earlier.11,28-31 Most relevantly in the context of the present study, no differences can be observed, within the experimental scatter, between the shearforce traces taken with torn-off and with melt-cut mica. Forces were also measured between mica surfaces, prepared in different ways, across cyclohexane, a liquid of quasi-spherical molecules known to form layers between mica surfaces at separations closer than a few molecular diameters.23,32 In view of the reported effects that the demonstrated presence of Pt nanoparticles had on the layering both of cyclohexane14 and other simple organic liquids,14,20 experiments comparing the behavior of such a liquid between melt-cut and torn-off mica surfaces are particularly revealing. Normal-force profiles were measured using torn-off and melt-cut mica sheets (in two laboratories). The results, which were based on two separate experiments using torn-off mica, shown as filled squares and filled triangles, and several separate experiments for melt-cut mica (open circles), as earlier reported,23 are given in Figure 7. They show clearly the characteristic oscillating or structural forces (28) Klein, J.; Raviv, U.; Perkin, S.; Kampf, N.; Chai, L.; Giasson, S. J. Phys.: Condens. Matter 2004, 16, S5437-S5448. (29) Leng, Y.; Cummings, P. T. Phys. ReV. Lett. 2005, 94, 026101. (30) Leng, Y.; Cummings, P. T. J. Chem. Phys. 2006, 124, 074711. (31) Sakuma, H.; Otsuki, K.; Kurihara, K. Phys. ReV. Lett. 2006, 96, 046104. (32) Christenson, H. J. Chem. Phys. 1983, 78, 6906-6913.

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Figure 7. Normal-force vs distance F(D)/R profiles between mica surfaces across cyclohexane. Filled symbols are for torn-off mica (based on two separate experiments, filled squares and filled triangles, Toronto). Open circles are for melt-cut mica (based on results (Weizmann) adapted from ref 23), and white triangles are the extrema from the force-distance profile for cyclohexane confined between melt-cut mica sheets from the study by Christenson.32 Each of the filled symbols (torn-off mica surfaces) represents either a repulsive maximum (jump-in) or an attractive minimum (jump-out) from a separate approach or separation run. The inset focuses on the data for torn-off mica only, showing also (as dotted-dashed curves) the envelopes of repulsive maxima and attractive minima from the meltcut mica data (main figure).

on approach and separation and are again very similar, whether torn-off or melt-cut mica is used, within the scatter. Shown in Figure 7 are the characteristic repulsive maxima on approach (when surfaces move spontaneously to the adjacent closer-in repulsive shoulder) and the jump-out points defining the minima of the oscillating forces. Each point shown for each of the two torn-off mica experiments (filled squares and triangles) represents a separate jump-in or jump-out position (corresponding to a separate in-out force profile). Also shown in Figure 7 (open triangles) are the extrema (either the tops of repulsive maxima or jump-out points defining the attractive minima) for cyclohexane confined between melt-cut mica surfaces taken from a much earlier study.32 Shear forces were then measured between the surfaces across the cyclohexane at progressively smaller separations. In Figure 8 are shown the shear traces corresponding to the motion of the lower surface at different surface separations D, in response to the motion of the upper surface. In Figure 8a and b, we illustrate the sharp transition in the behavior of the liquid as its confinement changes by a single molecular spacing, comparing results using torn-off mica with those obtained using melt-cut mica. Figure 8a (torn-off mica) shows the response in the absence of any applied motion of the upper surface: the vibrations correspond to the lateral motion of the lower surface that is excited by the ambient noise. One sees clearly that these vibrations, which are of similar magnitude from large separations down to D ) 3.8(0.2 nm (corresponding to n ) 7 layers of cyclohexane), become abruptly dampened when the surfaces approach by another single monolayer to D ) 3.2 ( 0.2 nm (n ) 6). This torn-off mica behavior (Figure 8a) is qualitatively the same as that previously observed23 both for cyclohexane and for OMCTS (octamethylcyclotetrasiloxane) confined between (melt-cut) mica sheets; the behavior of the latter is reproduced in Figure 8b on the basis of ref 23. The sudden dampening of the vibrations seen in Figure 8a is attributed to an abrupt increase in the effective viscosity of the confined cyclohexane as n ) 7 w n ) 6 (as observed

Langmuir, Vol. 22, No. 14, 2006 6147

Figure 8. (a) Traces showing lateral vibrations of the bottom mica surface due to ambient noise (no applied lateral motion of the top mica surface) at three separations D across cyclohexane using tornoff mica sheets (Toronto). At large separation (top trace) down to D ) 3.8 ( 0.2 nm (n ) 7 monolayers), the vibrations are large and essentially of the same magnitude. On reducing D by a single molecular spacing to 3.2 ( 0.2 nm (n ) 6), the vibrations become significantly dampened, indicating that the effective viscosity of the confined cyclohexane has abruptly increased at this transition. (b) The same as for part (a) but for confined OMCTS between melt-cut mica sheets, (Weizmann) taken from ref 23. Although (a) and (b) show the same qualitative damping of oscillations as n goes from 7 to 6, the differences in detail arise both from the difference in time scales and also from the larger vibrations in the current experiments (a). (c) Traces showing force transmitted to the bottom mica surface across the confined cyclohexane in response to a back-and-forth lateral motion of the top mica surface (for torn-off mica sheets) (Toronto). For surface separation corresponding to n ) 7 (D ) 3.9 ( 0.2 nm), LHS of part (c), there is no response that is different to the ambient vibration level seen in part a, indicating the fluidity of the confined liquid. On approach by another single molecular spacing to n ) 6 (D ) 3.2 ( 0.2 nm), the response of the bottom surface (RHS of part c) shows that the confined cyclohexane has abruptly become solidlike: it is capable of supporting a shear stress (part a of the trace) up to point b when a yield stress is attained and the surfaces slide past each other (stick-slip motion in the lower traces). The shear (or frictional) force at the yield point is given by half of the plateau-plateau separation (ca. 4 µN for the trace shown).

earlier for confined OMCTS, Figure 8b). Finally, the traces in Figure 8c show, for torn-off mica sheets, the shear forces transmitted to the lower mica surface in response to a backand-forth lateral motion of the upper surface across a gap of thickness D between the mica surfaces. For D ) 3.8 ( 0.2 nm, corresponding to n ) 7 cyclohexane monolayers, the response is similar to that of ambient noise in Figure 8a: no measurable shear force is transmitted across the confined liquid. On reducing the film thickness by a single monolayer, from n ) 7 to 6 monolayers, D ) 3.2 ( 0.2 nm, the response abruptly becomes characteristic of a solidlike material: a shear stress is sustained up to some value beyond which sliding motion occurs. This is very similar to the behavior across cyclohexane and OMCTS confined by melt-cut mica surfaces, which at the n ) 7 to 6 transition abruptly become capable of sustaining a shear stress, as described and discussed in detail elsewhere.23,33 Finally, we investigated the issue of the formation on freshly cleaved mica surfaces of an adsorbed layer a few angstroms thick due to airborne molecules8,9 following short exposure to (33) Cui, S. T.; Cummings, P. T.; Cochran, H. D. J. Chem. Phys. 2001, 114, 7189-7195.

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Figure 9. (Left) Picture (Oxford) of the FECO pattern across the bifurcation line (as indicated on the photo with an arrow and shown on the schematic diagram as B) for a mica sheet that has been cleaved halfway, opened, and exposed to ambient air in a laminar flow hood for 2 min and then closed again, as shown on the right. The doublet fringe pattern is seen for two adjacent fringes (the pth and (p - 1)th fringes transmitting FECO at wavelengths of λ0p and λp0 - 1 through the uncleaved portion of the mica sheet). The doublet on the right is the mercury yellow doublet used for calibration (wavelengths: 5790.65 and 5769.60 Å). The step δλ in wavelength at the bifurcation line relative to the wavelengths λ0p, arising from a double layer of thickness 2c adsorbed on the mica surfaces during the 2 min exposure and trapped between them on closing, is seen clearly in the photograph and is indicated (by an arrow that is slightly magnified for clarity) at the bottom. The thickness of the adsorbed double layer is related to δλ as follows: 2c ) [λp0 - 1/(λp0 - 1 - λ0p)][δλ/2µmica(λ0p)], where µmica(λ0p) is the (known) refractive index of mica at wavelength λ0p. From the values of the wavelengths λp0 - 1 and λ0p and step δλ, we readily obtain the thickness 2c ) 1.1 ( 0.4 nm (with the estimated error arising from the uncertainty in δλ). A more detailed calculation taking into account the asymmetry leads to a mean value for 2c in the same range.

ambient air, as described above. Figure 9A shows a photograph of the interferometric FECO patterns across the bifurcation line (the line up to which the cleavage was initiated) following partial cleavage of a 9-10-µm-thick mica sheet, exposure of the freshly cleaved surfaces to ambient air for 2 min, and then closing up again of the cleaved region. The procedure is shown in the cartoon in Figure 9B. The RHS of the picture of the fringes shows the mercury yellow doublet lines, whereas in the interferometric fringes themselves a small-wavelength step at the bifurcation line is clearly seen (magnified schematically at the bottom of the photo), corresponding to the thin layer of adsorbed film that formed within the 2 min that the freshly cleaved mica was exposed to air. The thickness 2c of this layer is evaluated from the fringes (caption to Figure 9) and the step change δλ in the transmitted FECO wavelength and is 2c ) 1.1 ( 0.4 nm.

Discussion and Conclusions The main aim of the present work was to compare the behavior of aqueous and organic solvents confined to molecularly thin films between mica surfaces prepared in different ways: either using mica that had been melt-cut with a hot Pt wire via recommended procedures or using mica that had been torn off of a freshly cleaved facet that had not been exposed to platinum in any way. Before considering our main findings, we note the data of Figure 1, which confirms several earlier observations12-16 that the extended exposure of mica to hot Pt wires can result in the deposition of Pt nanoparticles. The density and characteristics of these depend on the length and diameter of the wire, its temperature, distance from the wire, and other factors (such as the extent of air flow during the melt cutting). It is of interest to estimate the radius RL of the “lens” formed when an isolated nanoparticle is trapped between two mica surfaces of thickness

t in adhesive contact in air. This is done in the Appendix, where it is shown that for a particle of height h, RL is given by

RL4 )

h2t3E {12(1 - ν2)γ}

(1)

where E is the modulus of the mica, ν is its Poisson’s ratio, and γ is the mica/air interfacial energy. Using typical values (Appendix), we find the diameter of the lens formed by a trapped nanoparticle (taking h ) 5 nm) to be 2RL ≈ 4 µm. A single such lens in the contact area projected onto the spectrometer slit in the SFB experiment might be revealed in the shape of the interferometric fringes, although this is at the limit of optical resolution in the SFB. The central finding of this study is that surface force experiments on confined liquids using mica surfaces prepared by melt cutting according to the procedure in our different laboratories (i.e., thin, short Pt wires, a rapid cut time, and downstream handling as far as possible, as in Figure 2) yield results that are identical, within the experimental scatter, to those obtained using torn-off mica sheets. This applies whether the confined liquid is highly purified (conductivity) water with no added salt, whether it is a concentrated aqueous salt solution, or whether it is a typical organic solvent (cyclohexane) that layers under confinement. AFM micrographs (Figures 2 and 3) also reveal that mica surfaces melt cut in this way appear to be essentially free of any Pt nanoparticles away from the melt-cut edges, as are the torn-off mica surfaces. Conductivity Water (No Added Salt). Examining the results in more detail, we first consider normal forces across conductivity water (Figure 4). These differ slightly in the decay length of the repulsive interaction (Debye length) because of small differences in the effective salt concentration in the range of 10-6-10-5M, attributable to slightly different water purification and ion-leaching levels, and in the repulsive hump just prior to the jump-in, which

Forces between Mica Surfaces

varies somewhat because of slight differences in the mica surface potential. (Mica is a naturally occurring material and may have some slight compositional variation between samples.) All of the profiles, however, whether using torn-off or melt-cut mica, show the same main features, which are well-understood in terms of classic DLVO theory.34-36 They all reveal a weak, longranged osmotic repulsion, followed by a jump-in, driven by van der Waals attraction, from a spring-instability point at separations Dj ) 4 ( 2 nm to adhesive contact at a separation that is in the range of 0.8 ( 0.5 nm closer in than for air contact. The latter observation reveals that the thin, air-adsorbed contaminant layer noted earlier, which is present on both the melt-cut and on the torn-off mica (see below), is removed, as previously observed,2,10,26 on immersion in water. The profiles using melt-cut mica in Figure 4, prepared and measured independently by five of the coauthors in two different laboratories, are all very similar within the scatter (and within the literature variations of surface potentials and effective low ionic concentrations) to the torn-off mica data. Pull-off forces (caption to Figure 4) yield micawater surface energies that are similar for both torn-off and meltcut mica and are within the range of literature values.24 The shear behavior between mica surfaces across water confined to progressively thinner layers (Figure 5) is again very similar, both qualitatively and quantitatively, whether melt-cut or torn-off mica is used. As reported earlier,26 as the surfaces slowly approach to contact under thermal drift, no shear forces above the noise level are detected nor are any noted as the surfaces jump into contact from Dj. This is an especially sensitive test because the upper limit of the shear forces prior to jump-in using both melt-cut and torn-off mica surfaces, as revealed by the frequency analyses on the right-hand sides of Figures 5 and 6, are within the resolution of (30 nN even when the surfaces are only 2 nm apart, a separation smaller than the height of any expected nanoparticle. One may estimate the force necessary to shear a single nanoparticle: the yield stress15,37 of Pt in such nanoparticles is on the order of 1.5 × 108 Nm-2; taking the lateral dimension of a nanoparticle as 25 nm, the force required to shear it is around 100 nN. This is significantly larger than the upper limit of the forces actually measured,38 some 30 nN, when shearing the mica surfaces across 2-nm-thick water layers (Figure 5). This demonstrates, assuming that the shear leads to plastic deformation of the Pt nanoparticle rather than to interfacial sliding, that even a single nanoparticle cannot be present over the effective interaction area (some 50 µm2) on the melt-cut mica surfaces because its presence would have been revealed by a shear force much greater than any actually measured. Aqueous NaCl Solutions. A comparison of the interactions across concentrated aqueous NaCl solutions (Figure 6) reveals that both normal and shear interactions between melt-cut and torn-off mica surfaces are very similar, indeed the same within the scatter. For the case of concentrated salt solutions such as 0.06 ( 0.02 M NaCl shown in Figure 6A, the hydration repulsion is short-ranged, and no jump-in to adhesive contact occurs.25 Although normal-force profiles are very similar for both meltcut and torn-off mica, the fact that the range of repulsion is comparable with the height h ≈ 3 nm of typical Pt nanoparticles (34) Derjaguin, B. V.; Landau, L. Acta Physiochim. (USSR) 1941, 14, 633662. (35) Verwey, E. J. W.; Overbeek, J. T. G. Theory of the Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (36) Safran, S. A., Statistical Thermodynamics of Surfaces, Interfaces and Membranes; Addison-Wesley: New York, 1994. (37) ASM Handbook Properties and Selection: Nonferrous Alloys and Special Purpose Materials; ASM International: 1991; Vol. 2. (38) This upper limit is 30 nN at the drive frequency, when systematic effects at large D values arising from coupling to the piezo wires are taken into account (Figures 5 and 6).

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gives one pause. In this situation, the shear forces (Figure 6B and C) are especially revealing. In the traces shown for melt-cut mica surfaces (Figure 6B), the upper limit of the shear forces at D ) 0.8 ( 0.3 nm (once systematic effects calibrated at large surface separations are subtracted; see frequency analysis panel for the shear force trace of Figure 6B) is within the resolution of (30 nN of the SFB and so is significantly smaller than the ca. 100 nN shear force (see above) that shearing even a single Pt nanoparticle would entail. The shear-force traces in Figure 6C using torn-off mica are very similar (also within the upper limit of 30 nN) and confirm the striking molecular-ball-bearing-like behavior of hydrated monovalent ions reported earlier.11,28,30,31 This effect, which will not be discussed further here, arises from the combination of the strong time-average attachment of the hydration layers to their host ion, together with their rapid exchange rate.28 Most relevantly in the context of the present study, their close similarity to the results using melt-cut mica clearly confirms the absence of any effect of Pt (either as nanoparticles or as a submonolayer of atoms) in the latter. The fact that our results with torn-off mica are at the same time closely similar to the large earlier body of work on interactions between melt-cut mica across water and aqueous salt solutions2,10,22,25,27,39-41 points to the absence of any effect of Pt contamination in those earlier studies. These conclusions may shed light on some of the earlier work on the shear of comparable concentrated aqueous salt solutions,42 the results of which differ from our present (and other11,29-31) findings. In that study,42 high apparent viscosities were reported for aqueous salt solutions confined between mica surfaces already at separations as high as 2.5 nm. Such separations (2.5 nm) are comparable with the reported height h of typical Pt nanoparticles resulting from melt cutting in the same laboratory:15 it is possible that the high viscosities at surface separations similar to h reported in that study42 are attributable to the effects arising from the presence of such nanoparticles. Cyclohexane. The normal interactions for both torn-off and melt-cut mica across cyclohexane, as revealed in Figure 7, are very similar within the scatter. They both show in particular the oscillating forces, extending to some seven molecular layers, with similar magnitudes and a periodicity of 0.55 ( 0.1 nm, and an asymmetric repulsion/attraction envelope (with repulsive humps being higher than attractive wells, as seen earlier for both cyclohexane and OMCTS as well as other confined organic liquids23,32,43-48). Looking in more detail at the structural forces, we note that the repulsive humps using the melt-cut mica (open circles) are systematically slightly higher than those with the torn-off mica (filled triangles and squares), though still within the scatter. We should recall in this connection that the heights of minima and maxima of the oscillating forces are sensitive to small differences in the level of humidity in the confined liquid,32 and the small differences in the height of the repulsive maxima between the different experiments may be attributed to this. They may also be sensitive to different levels of vibrations arising from ambient noise or from differing efficiency of the vibration (39) Klein, J.; Luckham, P. Macromolecules 1984, 17, 1041-1048. (40) Luckham, P.; Klein, J. J. Chem. Soc., Faraday Trans. 1 1984, 80, 865878. (41) Toprakcioglu, C.; Klein, J.; Luckham, P. 1987, 83, 1703-1709. (42) Zhu, Y.; Granick, S. Phys. ReV. Lett. 2001, 87, 096101-096104. (43) Christenson, H. Chem. Phys. Lett. 1985, 118, 455-458. (44) Christenson, H. K.; Blom, E. C. J. Chem. Phys. 1987, 86, 419-424. (45) Christenson, H.; Horn, R.; Israelachvili, J. J. Colloid Interface Sci. 1982, 88, 79-88. (46) Horn, R.; Israelachvili, J. J. Chem. Phys. 1981, 75, 1400-1411. (47) Marra, J.; Hair, M. L. J. Colloid Interface Sci. 1988, 125, 552-560. (48) Vanderlick, T. K.; Scriven, L. E.; Davis, H. T. Colloids Surf., A 1991, 52, 9.

6150 Langmuir, Vol. 22, No. 14, 2006

isolation (see comment in caption to Figure 8). The depths of the minima for the two types of mica are very similar for both torn-off and melt-cut mica. It is also of interest that the results from the early study by Christenson32 (open triangles indicating the extrema of the structural forces in his profiles), using meltcut mica across cyclohexane, are similar in magnitude both to the repulsive maxima and to the minima of the data using the torn-off mica. In the inset to Figure 7, we focus on the oscillating forces profile for the torn-off mica. We note in particular the good fit of the torn-mica extrema to the envelope of the maxima and minima from the studies on confined cyclohexane using the melt-cut mica23,32 as well as the asymmetry in the repulsion/ attraction forces noted in other studies on organic liquids.23,32,43-49 The lateral forces between torn-off mica surfaces across cyclohexane are also revealing. Both the abrupt damping of lateral motion due to ambient noise as the cyclohexane confinement goes from n ) 7 to 6 monolayers (Figure 8a) and the sudden appearance of a yield stress characteristic of solidlike behavior at that same transition (Figure 8c) for torn-off mica are in accord with the abrupt, confinement-induced solidification observed in our earlier studies for cyclohexane and other liquids confined between melt-cut mica sheets.23 We may compare in greater detail the actual yield stress σy required to slide across the cyclohexane at the onset of solidlike behavior, n ) 6 monolayers, as revealed in Figure 8c. The yield stress corresponds to the yield force for sliding, Fs,y (≈ 4 µN, from Figure 8c), per unit area being sheared: σy ) Fs,y/A, where A is the contact area (in the absence of applied load) corresponding to the adhesive minimum at n ) 6 monolayers (D ) 3.3-0.4 nm, Figure 7). Using the JKR contact mechanics model, we evaluate50 A ) (1.1 ( 0.3) × 10-10 m2, giving σy ) (3.6 ( 1) × 104 Nm-2. This result for the torn-off mica is the same, within the scatter, as the value σy ) (3 ( 1) × 104 N m-2 for the yield stress using melt-cut mica confining cyclohexane to n ) 6 monolayers (from Figure 8 in ref 51). Similar values of the yield stress may be evaluated for both torn-off and melt-cut mica surfaces also at a confinement of n ) 5 monolayers. We conclude that, at all points of comparison, the magnitude and range of structural forces, the asymmetry of the repulsive versus attractive extrema, the abrupt confinement-induced solidification at n ) 6 monolayers, the magnitude of the yield stress at the point of solidification, and the behavior of cyclohexane confined by Pt-free (torn-off) mica are identical, within experimental scatter, to the same parameters of cyclohexane confined between melt-cut mica in our laboratories, as reported earlier.23,51 Because our results on structural forces exhibited by confined cyclohexane and other organic liquids23,51 are in line with the large body of earlier work on these liquids,32,43,44,46,48,49 the fact that we obtain identical behavior using both melt-cut and tornoff (Pt-free) mica surfaces shows by comparison that these earlier studies were also not affected by Pt contamination. In particular, it implies, as also pointed out recently by others,49 that there is little call for any “reassessment”20 of the earlier body of work on structural forces across cyclohexane or other analogous simple fluids. We should note also that for the water, aqueous salt solution, and cyclohexane experiments we did not control for or systematically vary the relative orientation of the interacting mica sheets (though we did attempt to maximize the birefringence (49) Akbulut, M.; Maeda, N.; Israelachvili, J. Langmuir 2006, 22, 23972398. (50) A ) πa2, where for zero applied load and a pull-off force Fp the JKR model gives for a, the flattened radius, a3 ) 4RFp/K, with R ≈ 1 cm being the mica curvature radius and K ) (1 ( 0.3) × 109 N/m2 being the effective mica/glue modulus. Putting in values for Fp at n ) 6 (D ) 33Å) from Figure 8a, we obtain A ) (1.1 ( 0.3) × 10-10 m2. (51) Klein, J.; Kumacheva, E. J. Chem. Phys. 1998, 108, 7010-7022.

Perkin et al.

of the FECO doublets in order to improve the wavelength resolution). The fact that our results, from several independent experiments, were all similar within the scatter suggests that the effect of relative orientation across liquids does not play a major role in the interaction of the surfaces across these liquids. This is less surprising for the case of aqueous liquids where even very highly confined films are rather fluid, but is perhaps more unexpected for the case of cyclohexane following its confinementinduced solidification. Nonetheless, this observation is in agreement with recent measurements49 on frictional forces across highly confined OMCTS, a model “layering” organic liquid, where there was little effect of the relative surface orientation on the magnitude of the sliding friction. The agreement of our results using torn-off mica with results of other laboratories studying structural forces using melt-cut mica across organic liquids, may shed light on some of the discrepancies in the literature. One example is the case of confined alkanes, where the study by Christenson et al.52 shows oscillating forces and an attractive regime (for dodecane) at surface separations of D ) 2.5-3 nm, whereas a subsequent report53 on the same liquid showed significant repulsion at this separation. Because corresponding studies on cyclohexane32 in Christenson’s laboratory are in agreement with our new results using Pt-free mica (and so, by implication, are themselves free of the effect of Pt contamination), the discrepancy between the two results may be attributable15 to the presence of Pt nanoparticles (also of a typical height of 3 nm) in the study by Hu et al.53 In our experiments, we utilized the tear-off approach (Figure 3) to create a symmetric, Pt-free sandwich of two atomically smooth mica sheets for the surface-force experiments. In other experiments,19,20 the use of tape-peeling to create freshly cleaved Pt-free mica surfaces for surface-force measurements has been advocated. The tape-peel method comprises cleaving the top layer of mica from the preglued samples by attaching a piece of sticky tape to the surface and pulling it away.19 In the original report,19 tape-peeling was carried out and adhesion was measured in a sealed N2-filled glovebox with no exposure to ambient air. However, in subsequent surface-force experiments20,21 the tapecleaved surfaces were exposed to ambient air prior to adding liquid. There are a number of obvious intrinsic drawbacks in the tape-peel method. First, the mica sheet is more likely to have steps on it. Second, the two mica sheets used in the SFB experiment will have differing thicknesses, which complicates the analysis of the results (and, if the spectroscopic field of view does not capture a sufficient part of the spectrum, leads to significant absolute uncertainties in the measured surface separation20). Third and most importantly, there is a real risk of contamination (by polymers and other species diffusing into the region where interactions are measured) when the sticky cellulose strip is stuck onto the clean mica surfaces prior to ripping them apart. These may be acceptable risks if there is reason to believe that tape-peeled mica benefits from a significantly lower level of airborne contamination than mica prepared in other ways (e.g., torn-off mica), because of its shorter exposure time to ambient air. As shown explicitly in Figure 9, however, the airadsorbed layer on mica whose surface was freshly cleaved and exposed to the ambient atmosphere for as little as 2 min, a period shorter than the minimum air-exposure time required in surface force experiments,54 has a thickness of c ) (1.1 ( 0.4)/2 ≈ (52) Christenson, H. K.; Gruen, D. W. R.; Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1987, 87, 1834-1841. (53) Hu, H.-W.; Carson, G. A.; Granick, S. Phys. ReV. Lett. 1991, 66, 27582761. (54) This minimum time is necessary to calibrate the mica contact thickness in air prior to adding any liquid, irrespective of the mica preparation technique.

Forces between Mica Surfaces

0.4-0.8nm on each surface. This is very similar to the thickness of the layer adsorbed on mica after an hour’s exposure following cleaving1,2 and is comparable to the adsorbed layer thickness when using melt-cut or torn-off mica. This is seen not only from the direct measurement in Figure 9 and similar measurements earlier1,2 but also in the fact that in water, which is known to dissolve the airborne layer, both torn-off and melt-cut mica surfaces that jump into adhesive contact on approach do so to contact positions that are closer in than air contact by a separation very similar to 2c (Figure 4). The rapid coverage of freshly cleaved mica by airborne molecules in ambient air should not be surprising. In 1930, Obreimov55 demonstrated, in the course of experiments measuring the surface energy of mica, that within a very short time (on the order of seconds or less) the surface energy of mica cleaved in air dropped sharply relative to the surface energy of vacuum-cleaved mica, indeed to a value characteristic of long exposure to air. For these reasons, the extent of contamination in ambient air for the tape-peeled and for the torn-off or melt-cut mica (as carried out in our and other laboratories) are likely to be essentially identical given even a very short exposure of the tape-cleaved surfaces to ambient air. In summary, we conclude from this study that the melt cutting of mica sheets, far from leading as a general rule to Pt nanoparticle contamination affecting surface-force measurements,15,20 routinely results, when properly carried out, in interacting surfaces that are quite free of any nanoparticle deposition. In this article, we confirm recent reports by Israelachvili et al.16 and Raviv et al.18 Our measurements, however, go beyond this. We show that for a variety of generically different liquids (pure water, concentrated salt solutions, and a typical layering organic liquid such as cyclohexane) the normal and shear forces between the surfaces confining them are identical, within experimental scatter, whether the mica is torn off (and thus guaranteed to be free of Pt nanoparticles) or melt cut following our standard procedures. In view of the recent discussion in the literature concerning the possible presence of Pt nanoparticles and their effect on surface forces,12-16,20 it is appropriate to spell out some implications of the present study. Our results with torn-off mica agree not only with our own new and earlier melt-cut mica results but also with the large body of much earlier work (e.g., curves in Figures 4, 6, and 7) by others on surface interactions across water and salt solutions2,10,22,25,27,39-41 as well as in layering organic solvents.23,32,43-49It is therefore a safe extrapolation, by comparison with the Pt-free data, that melt cutting in these earlier studies (and in the different laboratories involved) also resulted in surfaces that were free of the effects of Pt contamination, either as nanoparticles or otherwise. Acknowledgment. We thank Ken Johnson for helping with the calculations in the Appendix and Jacob Israelachvili for useful suggestions regarding the partial cleaving method (Figure 9). We thank the EPSRC (U.K.) and the Israel Science Foundation for their support of this work.

Appendix: Particle Trapped between Mica Sheets Figure A1 is a schematic representation of a Pt nanoparticle trapped between two mica sheets glued to lenses. If the size of the particle (h) is much less than the thickness of the glue, then we can assume that the uniform pressure P applied to the glue is transmitted as a uniform pressure P to the back of the mica sheets. Each sheet then acts like a circular flat plate of radius RL subjected to a uniform pressure P on one face and a concentrated central load W on the other. (55) Obreimoff, J. W. Proc. R. Soc. London, Ser. A 1930, 127, 290-297.

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Figure A1.

Figure A2.

The deflection of the plate due to P is56

δP )

3πPRL2 (1 - ν2) (RL2 - r2)2 E 16πt3 R 2

(A1)

L

and the deflection due to W is

δW )

[

( )]

2 RL 3W (1 - ν ) 4(RL2 - r2) - 8r2 log 3 E r 16πt

(A2)

where E is the modulus of the mica. At the point of separation, r ) RL, the net curvature is zero

( ) ( ) d 2δ P dδ2

+

r)RL

d 2δ W dδ2

)0

(A3)

r)RL

which gives

2W ) πPRL2

(A4)

At the center, r ) 0, and δW - δP ) h/2, which leads to

RL2(1 - ν2) h [12W - 3πPRL2] ) 3 2 16πt E

(A5)

Hence, from eqs A4 and A5

RL4 )

8E ht3 2 3P(1 - ν )

(A6)

It is possible to account for the interfacial energy γ of each of the plates by using an energy approach to include an adhesive term. When this is done (K. L. Johnson, private communication), we obtain a modified equation for RL4

6152 Langmuir, Vol. 22, No. 14, 2006

RL4 )

1 8Eht3 3P(1 - ν2) x(1 + C/R 4) L

Perkin et al.

(A7)

where

C)

128Et3w 3P2(1 - ν2)

and w ) 2γ is the adhesion energy. In the case of mica sheets in adhesive contact in air due to van der Waals forces, with no external applied pressure P f 0. At this limit C/RL4 . 1, which eliminates P in the expression for RL4, giving

h2t3E RL ) 6(1 - ν2)w 4

(A8)

A qualitatively similar expression was presented, without

derivation, in refs 16 and 57. The interfacial energy deduced from the pull-off force24 is γ ) 0.12 J/m2 so that w ) 2γ ) 0.24 J/m2. Using a typical height of a nanoparticle h ) 5 nm, the modulus of mica E ) 1011 N/m2, and t ) 2 µm, we obtain RL ) 1.9 µm. Optimally, the lateral resolution (i.e., in the direction perpendicular to the mica separation distance) over the area of contact is ∼1 µm. Therefore, if a single nanoparticle is present in the area of contact then the bubble or lens surrounding it (diameter 2RL) may be observable during the experiment, though this is at the limit of the optical resolution of the SFB. However, if the density of nanoparticles increases above a certain threshold, where the lenses surrounding them begin to overlap, then there would be no mica-mica contact between the particles. This situation is indicated in Figure A2. LA053097H (56) Johnson, K. L., Contact Mechanics; Cambridge University Press: London, 2004. (57) Stengl, R.; Mitani, K.; Lehmann, V.; Gosele, U. Proc. IEEE SOS/SOI Technol. Conf. 1989, 123-124. (58) Chan, D.; Pashley, R.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283-285. (59) Israelachvili, J. J. Colloid Interface Sci. 1986, 110, 263-271.